The invention relates to an internal combustion engine having at least one combustion chamber, in which engine a surface of at least one component of the combustion chamber and/or of an internal-combustion-engine component which is close to the combustion chamber and carries the exhaust gas stream is at least partially coated with a catalytic coating, said surface coming into contact with an air-fuel mixture to be burned or with an exhaust gas stream. The invention further relates to PVD methods for producing such a coating.
The coating of various components in the combustion chamber of internal combustion engines with a plurality of coating materials having different properties is known from the state of the art.
For example, DE 101 30 673 A (EP 1 404 955 B) describes a catalytic coating of surfaces of the combustion chamber of internal combustion engines which comprises two oxidic components, namely a ternary vanadium oxide (in particular V2O5-XNX or V2O5-XCX) as a “basic catalyst” and a further metal oxide which is selected from oxides of cerium, lanthanum, rare earths and transition metals. The catalytic coating is to support the oxidation of coke residues of the combustion process and can be produced by means of various methods, such as plasma-enhanced and ion-enhanced vacuum treatment, in particular arc evaporation, chemical impregnation by the deposition of a metallic salt solution onto the basic catalyst layer and subsequent oxidation or by a sol-gel process. Details for performing the individual methods are not disclosed.
Coatings for piston heads which are to reduce fuel deposits and thus to improve the carburetion in the combustion chamber are known from DE 101 08 834 A. For this purpose, the coating is designed as a chemically inert and thus low-reactivity coating, as a coating with few fissures and/or anti-adhesive coating or as a coating with a low coefficient of friction. The materials used are TiN, TiAlN, ZrO2, Cr—CN, TiZr—CNOH, TiAl—CNOH, AlON, SiCH or Ni. PVD methods (physical vapor deposition) are generally mentioned as coating methods. The coatings exclusively aim at a modification of the physical property of the piston surface and have no chemically catalytic function.
DE 101 17 513 A (WO 02/081874 A) describes coatings on the groove and/or the valve bottom of intake valves, in particular of direct-injection spark-ignition engines, which are to prevent or reduce the coking of the intake valve. Heat-insulating ZrO2 coatings, microporous and/or anti-adhesive Cr—CN coatings, chemically inert TiZr—CHNO or TiAl—CHNO coatings and catalytic vanadium nitride (VN) or platinum coatings are disclosed.
Hultqvist et al. (A. Hultqvist, M. Christensen & B. Johanson “The Application of Ceramic and Catalytic Coatings to Reduce the Unburned Hydrocarbon Emissions from a Homogeneous Charge Compression Ignition Engine”, SAE Technical Series 2000-01-1833 (2000), 1-11) investigated the effects which thermal barrier layers (Al2O3) and catalytic coatings (Pt-doped ZrO2) of different layer thicknesses in the combustion chamber of an engine operated according to the HCCl method have on emissions. All coatings effected a reduction of CO emissions, and the catalytic ZrO2/Pt coatings surprisingly had a negative effect on HC emissions. In all, relatively thin thermal Al2O3 barrier layers produced the best emission results. The coatings were deposited according to the plasma method, i.e., according to a method belonging to the group of CVD methods (chemical vapor deposition).
DE 101 48 129 A describes catalytic coatings of combustion chamber surfaces of diesel engines which are to reduce the activation energy for fuel ignition and/or fuel combustion in order to effect a reliable auto-ignition even at low temperatures and/or pressures. The coatings contain a carrier layer made of SiO2 or soot-shaped carbon, on which carrier layer a catalytic component is deposited which is selected from metals of the subgroups, lanthanides or actinides and/or the oxides and alloys thereof. The catalytic component is preferably present in the form of particles, in particular nanoparticles. The coating can be deposited according to the PVD (physical vapor deposition) or CVD (chemical vapor deposition) methods or by means of wet-chemical impregnation and subsequent thermal baking.
EP 1 878 879 A discloses a turbocharger whose stream-carrying compressor part is provided with a catalytic coating which is to decompose oily contaminants from so-called blow-by gases and thus to counteract the settling and coking thereof. The catalytic coating produced by means of thermal spraying comprises at least one transition metal oxide or mixtures thereof, wherein particularly oxides of the alloys TiZrNi, AlFeCrCo and/or AlFeCuCr and optionally a component like aluminum oxide are used.
DE 10 2005 033 118 B4 describes an internal combustion engine with an aluminum cylinder and/or aluminum piston whose side facing the combustion chamber is converted by anodizing into aluminum oxide having a hexagonal lattice structure. The hexagonal lattice forms a tubular system which includes catalytic noble metal nanoparticles added prior to anodizing. The catalyst particles have a catalytic conversion function, i.e., they serve to convert exhaust gas contaminants.
A method for the coating of catalyst substrates with a catalytically active substance is known from DE 102 19 643 A, wherein a porous carrier structure is deposited in a first step and the catalytic substance is deposited by means of plasma treatment making use of the hollow-cathode effect in a second step, wherein the process parameters are selected such that nanogranular particles having diameters of between 1 and 100 nm are obtained.
Nowadays, catalytically active surfaces, in particular of catalytic converters, are usually produced by depositing a coating on the basis of an aqueous suspension of salts of the catalytic metals (so-called washcoat) onto a carrier substrate. The metallic salts used are converted into their catalytically active form (the elemental metal or a metal oxide) in a high-temperature step (calcination). In this step, the necessary mechanical stability of the coating is also achieved. Aside from the desired oxidation and structure formation, said calcination also includes sintering processes, i.e., agglomeration of catalytic material, whereby nanostructures which are at first present are destroyed partially or even completely. Catalytic activity is reduced because the destruction of the nanostructures results in a reduced specific surface area of the catalytic coating. Particularly with coated combustion chamber surfaces or surfaces which are close to the combustion chamber, the high operating temperatures also result in sintering effects in the course of time, whereby microstructures or nanostructures which are possibly present are destroyed.